Denying the holocaust ever happened is rather disgusting. The eye witnesses, some of them even alive today, and the undeniable testimony , evidence, video, large graves, make anyone trying to deny it fools. Those who try to debate the exact number, whether one or six million died are engaging in a pointless pursuit. One million killed in that fashion is just as evil.
If someone is trying to counter those who justify Israels occupation by claiming they have a right to because of the holocaust, that is fair, because the holocaust is a terrible human tragedy and one would insult those who were massacred if they used it to justify another injustice happening today. However, in doing so one must be clear that the holocaust and racism and antisemitism, can never be justified.